FIG. 3 is an electrical circuit diagram showing a conventional capacitor-operated spot welder indicated in a capacitor-operated spot welder catalog (May 1982) published by "Origin Denki Shoji Kabushiki Kaisha (Origin Electric Commercial Co. Ltd.)" in Japan. In FIG. 3, reference numeral 1 designates a charging and discharging electrolytic capacitor; 2, a back-to-back type thyristor switch for obtaining current from the capacitor 1; 3, a welding transformer for increasing the current; 4, materials to be spot-welded by the output of the welding transformer; 5, a DC source for charging the charging and discharging electrolytic capacitor 1; and 6, a charging resistor for suppressing the charging current.
The conventional spot welder is constructed as described above. Therefore, when, under the condition that the electrolytic capacitor 1 has been charged to a voltage V by the DC source 5, the thyristor switch 2 is turned on, the electrolytic capacitor 1 is discharged through the welding transformer 3 which is low in impedance, so that a current i having a large peak value flows. The parts (a) and (b) of FIG. 4 are explanatory diagrams showing the terminal voltage V of the electrolytic capacitor and the waveform of the discharge current, respectively. The components of the circuit have a loss. As a result, both the terminal voltage and the discharge current are of an oscillatory waveform which damps with time. The current, which is substantially proportional to the current i in the part (b) of FIG. 4 in accordance with the winding ratio of the welding transformer 3, is applied to the materials 4 through the welding transformer 3. As shown in the part (b) of FIG. 4, the oscillatory current i is interrupted at the current zero point t.sub.off after the thyristor switch is turned off. Thereafter, the electrolytic capacitor 1 is charged by the DC source 5 again; that is, the welding circuit is placed in the initial state. In the spot welder, the pulse-shaped oscillatory current is repeatedly caused to flow to spot-weld the materials.
However, the above-described conventional capacitor-operated spot welder suffers from difficulties that, since the terminal voltage of the electrolytic capacitor becomes negative as indicated by the shaded regions 10a and 10b in the part (a) of FIG. 4, the electrolytic capacitor 1 is deteriorated, so that the welding performance is gradually lowered.